From coverage to shadows: How nonlinear vegetation-building synergy reshape urban cooling

Urban overheating poses escalating threats to energy consumption, human health, and environmental sustainability in rapidly developing high-density cities. However, previous studies have inadequately addressed the three-dimensional shading interactions between vegetation and buildings that shape localized cooling effects. Therefore, this study develops a Horizontal-Roughness-Overshadow (HRO) framework to quantify vegetation-based cooling performance under varying building-shadow conditions at multiple times of day of ChongQing. Results reveal that although three-dimensional vegetation shading significantly alleviates urban overheating, denser canopies can impede airflow and induce threshold-dependent, nonlinear temperature feedbacks—evidenced by an early-morning GCE_V threshold of approximately 4 marking the switch from warming to cooling effects— with notable spatial heterogeneity showing that core urban areas exhibit distinct overshadow patterns compared to peripheral zones. These findings not only deepen theoretical understanding of vegetation's 3D cooling mechanisms but also offer tangible management strategies-such as balancing canopy density with ventilation and refining vertical greening layouts-for more efficient, context-specific heat mitigation.